1. Field of the Invention
This invention relates to a method for treating dispersions of two or more immiscible liquids to obtain a separation of the liquids one from the others.
More specifically, this invention relates to a method for treating dispersions of oleophilic liquids, typically hydrocarbons, with water to recover the oleophilic liquid and to produce a clarified water stream.
Large volumes of waste liquids comprising dispersions of oleophilic materials with water are generated in the normal course of industry and commerce. The oleophilic materials are most commonly hydrocarbons from such sources as crude oils and refined petroleum products but may include as well halogenated hydrocarbons, animal and vegetable fats and oils and the like. Dispersions of oils and greases in water include bilge and ballast water, refinery and industrial plant wastes resulting from hydrocarbon processing and from the lubrication, cleaning and degreasing of machinery and parts. Huge volumes of water having oils dispersed therein are generated in the production of crude oil. Water is commonly produced along with crude oil and, particularly in the latter stages of a field's life, the volume of water produced often greatly exceeds that of the crude.
These waste dispersions almost always require treatment to separate the oleophilic liquid from the water, sometimes for economic reasons alone, but usually to meet the requirements for waste disposal set by law and regulations. Such laws and regulations commonly require that effluents discharged into surface waters contain less than about 40 to 50 ppm oil and be totally free of an oil film sheen. A surface sheen starts to become visible at an oil concentration of about 25 gallons per square mile.
The maximum concentration of oils and other hydrocarbons in discharged effluents allowed by regulation is usually set at levels which will protect the quality of domestic water supplies and which will not harm aquatic life. Even minute quantities of oils and greases can cause troublesome odor and taste problems in domestic water supplies and produce scum lines on water treatment facilities, swimming pools and other containers. Oil films on surface waters destroy aquatic insect life, may interfere with reaeration and photosynthesis processes, and can coat the gill membranes of fish causing suffocation. Further, the presence of hydrocarbon solvents, greases and oils can upset or otherwise harm the operation of sewage treatment plants so the discharge of oil containing wastes into sewer systems is carefully regulated.
2. Description of the Prior Art
The prior art has developed a large number of approaches for the treatment of oil and other oleophilic liquid dispersions with water. Such known treatment processes include settling either in sedimentation vessels of various sorts or in lagoons, flotation, membrane separation or ultrafiltration, emulsion breaking using a variety of chemical agents, electrical dehydration, and centrifugation using both centrifuges and hydrocyclones. The most commonly used treatment processes for high volumes of waste dispersions include settling, flotation, centrifugation and combinations of these. Each of these processes have certain disadvantages and none are totally satisfactory.
The efficiency of settling processes is greatly dependent upon the difference in specific gravity between water and the oleophilic liquid and upon the droplet size of the dispersed phase liquid. These processes work best with large droplet dispersions of a relatively light, low viscosity oleophilic liquid, such as diesel fuel for example, in water. These same processes, however, are of little value in the rectification of stable, or tight, emulsions or in the separation of dispersed liquids having similar specific gravities.
Flotation processes are commonly used to strip dispersed oil droplets from water. Both dissolved-air flotation and induced-air flotation processes are used here. In dissolved-air flotation, the aqueous stream is contacted with air at high pressure causing air to dissolve in the liquid. The pressure on the liquid is then reduced which releases air bubbles that sweep oil droplets to the liquid surface. In induced-air flotation, air is drawn into a flotation cell by action of a rotor and is dispersed into the liquid as tiny bubbles. The bubbles tend to attach to oil droplets which are then carried to the surface of the water to form a foam or froth. The froth is skimmed from the liquid surface into collection launders for removal.
Centrifugation processes using either centrifuges or hydrocyclones generally are capable of removing most dispersed oils from water to acceptably low levels. Oil removal efficiency of these devices depend primarily upon the density difference between the oil and the water. Oil removal efficiency is also affected by oil droplet size with the larger droplets being more readily separated than are smaller droplets. Process temperature is a consideration as well as it affects both density and fluid viscosities. All centrifugation processes require a relatively high energy input for adequate separation. Centrifuges require sufficient power input to develop centrifugal forces through the mechanical rotation of the fluid adequate to effect a separation of the two liquid phases. Hydrocyclones usually consist of an upper cylindrical section having a tangential feed entry and a lower conical section having a bottom apex discharge opening. Pressurized liquid is introduced tangentially into the upper cylindrical portion of the device creating a downward spiralling action developing high centrifugal forces to effect the oil-water separation.
Hydrocyclones perform best in those circumstances where the oil-water dispersion is under pressure as from a flowing oil well. Proper operation of a hydrocyclone requires a pressure drop across the unit ranging from about 5 to about 20 Bars. If the dispersion is at a lower pressure than that required to adequately drive the hydrocyclone, then pumping of the dispersion is required. Pumping tends to decrease the efficiency of hydrocyclone separations as existing oil droplets are sheared into smaller droplets as they pass through pumps, valves and piping at high velocity. This consideration limits the practical usefulness of hydrocyclones for the treatment of many oil-water dispersions.
It is evident that the prior art approaches to the clarification of dispersions of water and oleophilic liquids often present practical and economic difficulties not adequately addressed by the existing technology.